Podcasted lectures offer students the chance to replay difficult parts of a lecture and therefore take better notes, says Dani McKinney, a psychologist at the State University of New York in Fredonia, who led the study.

"It isn't so much that you have a podcast, it's what you do with it," she says.

Launched less than two years ago, Apple's iTunes university offers college lectures on everything from Proust to particle physics to students and the public. Some universities make their lectures available to all, while others restrict access to enrolled students. Some professors even limit downloads to encourage class attendance, McKinney says.

To find out how much students really can learn from podcast lectures alone - mimicking a missed class - McKinney's team presented 64 students with a single lecture on visual perception, from an introductory psychology course.

Half of the students attended the class in person and received a printout of the slides from the lecture. The other 32 downloaded a podcast that included audio from the same lecture synchronised with video of the slides. These students also received a printed handout of the material.

Students who downloaded the podcast averaged a C (71 out of 100) on the test - substantially better than those who attended the lecture, who on average mustered only a D (62).

But that difference vanished among students who watched the podcast but did not take notes.Students who listened to the podcast one or more times and took notes had an average score of 77, McKinney says.